Implantable medical devices having multi-cell power sources

ABSTRACT

An implantable medical device includes a low-power circuit and a multi-cell power source. The cells of the power source are coupled in a parallel configuration. The implantable medical device includes both a low power circuit that is selectively coupled between the first and second cells and a high power output circuit that is directly coupled to the first and second cells in a parallel configuration. An isolation circuit is coupled to the first cell, the second cell and the low power circuit to maintain a current isolation between the first cell and the second cell at least during delivery currents having a large magnitude that are delivered to the high power output circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/047,136, filed on Sep. 8, 2014. The disclosure of the aboveapplication is incorporated herein by reference in its entirety.

The present application is related to co-pending and commonly-assignedU.S. patent application Ser. No. 14/695,264 (Atty. Docket No.:C00006865.USU2) which is entitled Multi-Primary Transformer ChargingCircuits for Implantable Medical Devices; U.S. patent application Ser.No. 14/695,309 (Atty. Docket No.: C00006923.USU2), which is entitledImplantable Medical Devices Having Multi-Cell Power Sources; U.S. patentapplication Ser. No. 14/695,447 (Atty. Docket No.: C00006924.USU2),which is entitled Multiple Transformer Charging Circuits for ImplantableMedical Devices; U.S. patent application Ser. No. 14/695,630 (Atty.Docket No.: C00006939.USU2), which is entitled Transformer-BasedCharging Circuits for Implantable Medical Devices; U.S. patentapplication Ser. No. 14/695,887 (Atty. Docket No.: C00007047.USU2),which is entitled Transthoracic Protection Circuit for ImplantableMedical Devices; and U.S. patent application Ser. No. 14/695,826 (Atty.Docket No.: C00008057.USU3), which is entitled Monitoring Multi-CellPower Source of an Implantable Medical Device, all of which are filedconcurrently herewith and all of which are incorporated herein byreference in their entireties.

FIELD

The present disclosure relates to body implantable medical devices and,more particularly to circuits and techniques implemented in animplantable medical device to provide an electrical therapeutic output.

BACKGROUND

The human anatomy includes many types of tissues that can eithervoluntarily or involuntarily, perform certain functions. After disease,injury, or natural defects, certain tissues may no longer operate withingeneral anatomical norms. For example, organs such as the heart maybegin to experience certain failures or deficiencies. Some of thesefailures or deficiencies can be diagnosed, corrected or treated withimplantable medical devices.

Implantable medical electrical leads are used with a wide variety ofthese implantable medical devices. The medical leads may be configuredto allow electrodes to be positioned at desired cardiac locations sothat the device can monitor and/or deliver stimulation therapy to thedesired locations. For example, electrodes on implantable leads maydetect electrical signals within a patient, such as anelectrocardiogram, in addition to delivering electrical stimulation.

Currently, ICD's use endocardial or epicardial leads which extend fromthe ICD housing through the venous system to the heart. Electrodespositioned in or adjacent to the heart by the leads are used for pacingand sensing functions. Cardioversion and defibrillation shocks aregenerally applied between a coil electrode carried by one of the leadsand the ICD housing, which acts as an active can electrode.

A subcutaneous implantable cardioverter defibrillator (SubQ ICD) differsfrom the more commonly used ICD's in that the housing and leads aretypically implanted subcutaneously such that the sensing and therapy areaccomplished subcutaneously. The SubQ ICD does not require leads to beplaced in the heart or in contact with the heart. Instead, the SubQ ICDmakes use of one or more electrodes on the housing, together with asubcutaneous lead that carries a defibrillation coil electrode and asensing electrode.

The implantable medical devices are typically battery powered and oftenutilize capacitors or other electrical charge storage components to holdan electrical output to be made available to a patient. Due to thenature of defibrillation therapy or other high voltage therapy, it isnot practical for the implantable medical device to supply the energyupon instantaneous demand by drawing from the power source. Instead,additional circuitry is provided to transfer and store the energy fromthe power source to accumulate a desired voltage level.

However, the placement of the SubQ ICD lead(s) and electrode(s) outsidethe heart presents a challenge to generating sufficient energy levelsthat are required to deliver appropriate therapy. As described herein,the present disclosure addresses the need in art to provide circuitryand techniques for generating appropriate electrical stimulation therapyin a SubQ ICD system.

SUMMARY

In accordance with aspects of this disclosure, circuits and techniquesimplemented in an implantable medical device are provided for generatingan electrical stimulation therapy from a multi-cell power source. Suchelectrical stimulation therapy exhibits an output having a highervoltage than the voltage available directly from the battery or a highercurrent than the current available directly from the battery.

In accordance with some embodiments, the implantable medical devicecomprises a high power output circuit for delivery of an electricalstimulation therapy, a first cell directly coupled to the high poweroutput circuit, a second cell directly coupled to the high power outputcircuit, the first and second cells being arranged in a parallelconfiguration, a low power control circuit directly coupled to thesecond cell, wherein the low power circuit receives a first level ofcurrent from the first and second cells, and an isolation circuitcoupled to the first cell, the second cell, and the low power controlcircuit, with the isolation circuit being configured to maintain acurrent isolation between the first cell and the second cell such that asecond level of current is delivered from both the first and secondcells to the high power output circuit, and the second level of currentis greater than the first level of current.

In further aspects of the embodiments of the present disclosure, theisolation circuit is configured to maintain a current isolation betweenthe first cell and the second cell during the delivery of charge fromboth the first and second cells to the high power output circuit.

In further aspects of the embodiments of the present disclosure, theisolation circuit comprises a resistive load.

In further aspects of the embodiments of the present disclosure, theresistive load comprises an impedance value between 10 ohms and 10000ohms.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent disclosure and therefore do not limit the scope of thedisclosure. The drawings are not to scale (unless so stated) and areintended for use in conjunction with the explanations in the followingdetailed description. Embodiments will hereinafter be described inconjunction with the appended drawings wherein like numerals/lettersdenote like elements, and:

FIG. 1 is a front view of a patient implanted with an implantablecardiac system;

FIG. 2 is a side view the patient implanted with an implantable cardiacsystem;

FIG. 3 is a transverse view of the patient implanted with an implantablecardiac system;

FIG. 4 depicts a schematic diagram of an embodiment of operationalcircuitry included in an implantable cardiac defibrillator of thecardiac system of FIGS. 1-3;

FIG. 5 illustrates an exemplary schematic diagram showing a portion ofthe operational circuitry of FIG. 4 in accordance with an embodiment ofthe disclosure;

FIG. 6 illustrates an exemplary schematic diagram showing a portion ofthe operational circuitry of FIG. 4 in accordance with an embodiment ofthe disclosure; and

FIG. 7 illustrates an exemplary schematic diagram showing a portion ofthe operational circuitry of FIG. 4 in accordance with an embodiment ofthe disclosure.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram of a patient 12 implanted with an exampleextravascular cardiac defibrillation system 10. In the exampleillustrated in FIG. 1, extravascular cardiac defibrillation system 10 isan implanted subcutaneous defibrillation system for purposes ofillustration.

Extravascular cardiac defibrillation system 10 includes an implantablemedical device such as implantable cardiac defibrillator (ICD) 14connected to at least one implantable cardiac defibrillation lead 16.ICD 14 of FIG. 1 is implanted subcutaneously on the left side of patient12. Defibrillation lead 16, which is connected to ICD 14, extendsmedially from ICD 14 toward sternum 28 and xiphoid process 24 of patient12. At a location near xiphoid process 24 defibrillation lead 16 bendsor turns and extends subcutaneously superior, substantially parallel tosternum 28. In the example illustrated in FIG. 1, defibrillation lead 16is implanted such that lead 16 is offset laterally to the left side ofthe body of sternum 28 (i.e., towards the left side of patient 12).

ICD 14 may interact with an external device 4 such as a patientprogrammer or a clinician programmer via a 2-way telemetry link. Such aprogrammer communicates with ICD 14 via telemetry as is known in theart. The programmer 4 may thereby establish a telemetry session with ICD14 to provide programs, instructions, parameters, data, and otherinformation to ICD 14, and to likewise receive status, data, parameters,programs, and other information from the ICD 14. Status informationreceived from the ICD 14 may include data about the remaining longevityof the power source (e.g., a battery) based on the amount of charge thathas thus far been delivered by the battery and consumed by the ICD 14 ascompared to when the battery was in the full-charged state (“batterycapacity”). Status information may also include an “Elective ReplacementIndicator” (ERI) to indicate when surgery must be scheduled to replaceICD 14. Status may also include an “End of Life” (EOL), which isactivated to signify end-of-battery life.

Defibrillation lead 16 is placed along sternum 28 such that a therapyvector between defibrillation electrode 32 and a second electrode (suchas a housing or can electrode 36 36 of ICD 14 or an electrode placed ona second lead) is substantially across the ventricle of heart 26. Thetherapy vector may, in one example, be viewed as a line that extendsfrom a point on the defibrillation electrode 32 to a point on thehousing or can electrode 36 of ICD 14. In another example,defibrillation lead 16 may be placed along sternum 28 such that atherapy vector between defibrillation electrode 32 and a housing or canelectrode 36 of ICD 14 (or other electrode) is substantially across anatrium of heart 26. In this case, extravascular ICD system 10 may beused to provide atrial therapies, such as therapies to treat atrialfibrillation.

The embodiment illustrated in FIG. 1 is an example configuration of anextravascular ICD system 10 and should not be considered limiting of thetechniques described herein. For example, although illustrated as beingoffset laterally from the midline of sternum 28 in the example of FIG.1, defibrillation lead 16 may be implanted such that lead 16 is offsetto the right of sternum 28 or over sternum 28. Additionally,defibrillation lead 16 may be implanted such that it is notsubstantially parallel to sternum 28, but instead offset from sternum 28at an angle (e.g., angled lateral from sternum 28 at either the proximalor distal end). As another example, the distal end of defibrillationlead 16 may be positioned near the second or third rib of patient 12.However, the distal end of defibrillation lead 16 may be positionedfurther superior or inferior depending on the location of ICD 14,location of electrodes 32, 34, and 30, or other factors.

Although ICD 14 is illustrated as being implanted near a midaxillaryline of patient 12, ICD 14 may also be implanted at other subcutaneouslocations on patient 12, such as further posterior on the torso towardthe posterior axillary line, further anterior on the torso toward theanterior axillary line, in a pectoral region, or at other locations ofpatient 12. In instances in which ICD 14 is implanted pectorally, lead16 would follow a different path, e.g., across the upper chest area andinferior along sternum 28. When the ICD 14 is implanted in the pectoralregion, the extravascular ICD system may include a second lead includinga defibrillation electrode that extends along the left side of thepatient such that the defibrillation electrode of the second lead islocated along the left side of the patient to function as an anode orcathode of the therapy vector of such an ICD system.

ICD 14 includes a housing that forms a hermetic seal that protectscomponents within ICD 14. The housing of ICD 14 may be formed of aconductive material, such as titanium or other biocompatible conductivematerial or a combination of conductive and non-conductive materials. Insome instances, the housing of ICD 14 functions as an electrode(sometimes referred to as a housing electrode or can electrode) that isused in combination with one of electrodes 32, 34, or 30 to deliver atherapy to heart 26 or to sense electrical activity of heart 26. ICD 14may also include a connector assembly (sometimes referred to as aconnector block or header) that includes electrical feedthroughs throughwhich electrical connections are made between conductors withindefibrillation lead 16 and electronic components included within thehousing. The housing may enclose one or more components, includingprocessors, memories, transmitters, receivers, sensors, sensingcircuitry, therapy circuitry and other appropriate components (oftenreferred to herein as modules).

Defibrillation lead 16 includes a lead body having a proximal end thatincludes a connector configured to connect to ICD 14 and a distal endthat includes one or more electrodes 32, 34, and 30. The lead body ofdefibrillation lead 16 may be formed from a non-conductive material,including silicone, polyurethane, fluoropolymers, mixtures thereof, andother appropriate materials, and shaped to form one or more lumenswithin which the one or more conductors extend. However, the techniquesare not limited to such constructions. Although defibrillation lead 16is illustrated as including three electrodes 32, 34 and 30,defibrillation lead 16 may include more or fewer electrodes.

Defibrillation lead 16 includes one or more elongated electricalconductors (not illustrated) that extend within the lead body from theconnector on the proximal end of defibrillation lead 16 to electrodes32, 34 and 30. In other words, each of the one or more elongatedelectrical conductors contained within the lead body of defibrillationlead 16 may engage with respective ones of electrodes 32, 34 and 30.When the connector at the proximal end of defibrillation lead 16 isconnected to ICD 14, the respective conductors may electrically coupleto circuitry, such as a therapy module or a sensing module, of ICD 14via connections in connector assembly, including associatedfeedthroughs. The electrical conductors transmit therapy from a therapymodule within ICD 14 to one or more of electrodes 32, 34 and 30 andtransmit sensed electrical signals from one or more of electrodes 32, 34and 30 to the sensing module within ICD 14.

ICD 14 may sense electrical activity of heart 26 via one or more sensingvectors that include combinations of electrodes 34 and 30 and a housingor can electrode 36 of ICD 14. For example, ICD 14 may obtain electricalsignals sensed using a sensing vector between electrodes 34 and 30,obtain electrical signals sensed using a sensing vector betweenelectrode 34 and the conductive housing or can electrode 36 of ICD 14,obtain electrical signals sensed using a sensing vector betweenelectrode 30 and the conductive housing or can electrode 36 of ICD 14,or a combination thereof. In some instances, ICD 14 may even sensecardiac electrical signals using a sensing vector that includesdefibrillation electrode 32, such as a sensing vector betweendefibrillation electrode 32 and one of electrodes 34 or 30, or a sensingvector between defibrillation electrode 32 and the housing or canelectrode 36 of ICD 14.

ICD 14 may analyze the sensed electrical signals to detect tachycardia,such as ventricular tachycardia or ventricular fibrillation, and inresponse to detecting tachycardia may generate and deliver an electricaltherapy to heart 26. For example, ICD 14 may deliver one or moredefibrillation shocks via a therapy vector that includes defibrillationelectrode 32 of defibrillation lead 16 and the housing/can electrode.Defibrillation electrode 32 may, for example, be an elongated coilelectrode or other type of electrode. In some instances, ICD 14 maydeliver one or more pacing therapies prior to or after delivery of thedefibrillation shock, such as anti-tachycardia pacing (ATP) or postshock pacing. In these instances, ICD 14 may generate and deliver pacingpulses via therapy vectors that include one or both of electrodes 34 and30 and/or the housing/can electrode. Electrodes 34 and 30 may comprisering electrodes, hemispherical electrodes, coil electrodes, helixelectrodes, segmented electrodes, directional electrodes, or other typesof electrodes, or combination thereof. Electrodes 34 and 30 may be thesame type of electrodes or different types of electrodes, although inthe example of FIG. 1 both electrodes 34 and 30 are illustrated as ringelectrodes.

Defibrillation lead 16 may also include an attachment feature 29 at ortoward the distal end of lead 16. The attachment feature 29 may be aloop, link, or other attachment feature. For example, attachment feature29 may be a loop formed by a suture. As another example, attachmentfeature 29 may be a loop, link, ring of metal, coated metal or apolymer. The attachment feature 29 may be formed into any of a number ofshapes with uniform or varying thickness and varying dimensions.Attachment feature 29 may be integral to the lead or may be added by theuser prior to implantation. Attachment feature 29 may be useful to aidin implantation of lead 16 and/or for securing lead 16 to a desiredimplant location. In some instances, defibrillation lead 16 may includea fixation mechanism in addition to or instead of the attachmentfeature. Although defibrillation lead 16 is illustrated with anattachment feature 29, in other examples lead 16 may not include anattachment feature 29. In this case, defibrillation lead 16 may beconnected to or secured to an implant tool via an interference fit aswill be described in more detail herein. An interference fit, sometimesalso referred to as a friction fit, is a fastening between two partswhich is achieved by friction after the parts are pushed together,rather than by any other means of fastening.

Lead 16 may also include a connector at the proximal end of lead 16,such as a DF4 connector, bifurcated connector (e.g., DF-1/IS-1connector), or other type of connector. The connector at the proximalend of lead 16 may include a terminal pin that couples to a port withinthe connector assembly of ICD 14. In some instances, lead 16 may includean attachment feature at the proximal end of lead 16 that may be coupledto an implant tool to aid in implantation of lead 16. The attachmentfeature at the proximal end of the lead may separate from the connectorand may be either integral to the lead or added by the user prior toimplantation.

Defibrillation lead 16 may also include a suture sleeve or otherfixation mechanism (not shown) located proximal to electrode 30 that isconfigured to fixate lead 16 near the xiphoid process or lower sternumlocation. The fixation mechanism (e.g., suture sleeve or othermechanism) may be integral to the lead or may be added by the user priorto implantation.

The example illustrated in FIG. 1 is exemplary in nature and should notbe considered limiting of the techniques described in this disclosure.For instance, extravascular cardiac defibrillation system 10 may includemore than one lead. In one example, extravascular cardiac defibrillationsystem 10 may include a pacing lead in addition to defibrillation lead16.

In the example illustrated in FIG. 1, defibrillation lead 16 isimplanted subcutaneously, e.g., between the skin and the ribs and/orsternum. In other instances, defibrillation lead 16 (and/or the optionalpacing lead) may be implanted at other extravascular locations. In oneexample, defibrillation lead 16 may be implanted at least partially in asubsternal location. In such a configuration, at least a portion ofdefibrillation lead 16 may be placed under or below the sternum in themediastinum and, more particularly, in the anterior mediastinum. Theanterior mediastinum is bounded laterally by pleurae, posteriorly bypericardium, and anteriorly by sternum. Defibrillation lead 16 may be atleast partially implanted in other extra-pericardial locations, i.e.,locations in the region around, but not in direct contact with, theouter surface of heart 26. These other extra-pericardial locations mayinclude in the mediastinum but offset from sternum 28, in the superiormediastinum, in the middle mediastinum, in the posterior mediastinum, inthe sub-xiphoid or inferior xiphoid area, near the apex of the heart, orother location not in direct contact with heart 26 and not subcutaneous.In still further instances, the implant tools described herein may beutilized to implant the lead at a pericardial or epicardial locationoutside the heart 26. Moreover, implant tools such as those describedherein may be used to implant non-cardiac leads in other locationswithin patient 12.

In an example, lead 16 may be placed in the mediastinum and, moreparticularly, in the anterior mediastinum. The anterior mediastinum isbounded laterally by pleurae 40, posteriorly by pericardium 38, andanteriorly by sternum 22. Lead 16 may be implanted within themediastinum such that one or more electrodes 32 and 34 are located overa cardiac silhouette of the ventricle as observed via fluoroscopy. Inthe example illustrated in FIGS. 1-3, lead 16 is located substantiallycentered under sternum 22. In other instances, however, lead 16 may beimplanted such that it is offset laterally from the center of sternum22. Although described herein as being implanted in the substernalspace, the mediastinum, or the anterior mediastinum, lead 16 may beimplanted in other extra-pericardial locations.

Electrodes 30, 32, and 34 may comprise ring electrodes, hemisphericalelectrodes, coil electrodes, helical electrodes, ribbon electrodes, orother types of electrodes, or combinations thereof. Electrodes 30, 32and 34 may be the same type of electrodes or different types ofelectrodes. In the example illustrated in FIGS. 1-3 electrode 34 is acoil electrode and electrodes 30 and 34 are ring, or hemisphericalelectrodes.

FIG. 4 is a schematic diagram of operational circuitry 48 included inICD 14 according to an embodiment of the present disclosure. It isunderstood that the system of FIG. 4 includes both low power circuitryand high power circuitry. The present disclosure may be employed in adevice that provides either or both of a high power electricalstimulation therapy, such as a high power defibrillation therapy, or alow power electrical stimulation therapy, such a pacing pulse, or both.Accordingly, the components in the operational circuitry 48 may supportgeneration and delivery of either one or both such therapies. For easeof description, this disclosure will describe an operational circuitry48 that supports only a high power electrical stimulation therapy, suchas cardioversion and/or defibrillation stimulation therapy. However, itshould be noted that the operational circuitry 48 may also providedefibrillation threshold (DFT) induction therapy or post-shock pacingsuch as anti-tachycardia pacing (ATP) therapy.

The operational circuitry 48 is provided with at least one or more powersource(s) 46 which may include a rechargeable and/or non-rechargeablebattery having one or more cells. As used in this disclosure, the term“cell” refers to a battery cell, which as is understood in the art,includes an anode terminal and a cathode terminal. An example of abattery cell is set forth in commonly assigned U.S. Patent ApplicationNo. US 2011/0179637 “Implantable Medical Devices with Low VolumeBatteries, and Systems”, to Norton which is incorporated herein byreference. As described in greater detail below, the power source 46 canassume a wide variety of forms. Similarly, the operational circuitry 48,which includes the low power circuit 60 and the output circuit 56, caninclude analog and/or digital circuits, can assume a variety ofconfigurations, and is electrically connected to the power source 46.

A power source monitoring circuit 62 is provided for monitoring themagnitude of residual energy and/or rate of depletion of energy from thepower source 46. The monitoring circuit 62 may monitor the power sourceby measuring a parameter that is, for example, indicative of theresidual energy or rate of discharge, of the power source 46. Themonitoring circuit 62 may employ techniques that involve computing theindication of the residual energy, or rate of discharge, of the powersource 46 (or individual cells) utilizing a parameter such as thevoltage across terminals of power source 46. In other embodiments,monitoring circuit 62 may alternatively or additionally have thecapability to measure a parameter such as current flowing from the powersource 46.

The output circuit 56 and the low power circuit 60 are typicallyprovided as part of an electronics module associated with the ICD 14. Ingeneral terms, the output circuit 56 is configured to deliver anelectrical pulse therapy, such as a defibrillation or acardioversion/defibrillation pulse. In sum, the output circuit 56 isresponsible for applying stimulating pulse energy between the variouselectrodes 28-34 (FIG. 1) of the ICD 14. As is known in the art, theoutput circuit 56 may be associated with a capacitor bank (not shown)for generating an appropriate output energy, for example in the range of0.1-40 Joules.

The low power circuit 60 is similarly well known in the art. In generalterms, the low power circuit 60 monitors heart activity and signalsactivation of the output circuit 56 for delivery of an appropriatestimulation therapy. Further, as known in the art, the low power circuit60 may generate a predetermined series of pulses from the output circuit56 as part of an overall therapy.

In an embodiment, ICD 14 functions are controlled by means of storedsoftware, firmware and hardware that cooperatively monitor the EGM,determine when a cardioversion or defibrillation shock necessary, anddeliver prescribed defibrillation therapies. The schematic diagram ofFIG. 4 incorporates circuitry set forth in commonly assigned U.S. Pat.No. 5,163,427 “Apparatus for Delivering Single and MultipleCardioversion and Defibrillation Pulses” to Keimel and U.S. Pat. No.5,188,105 “Apparatus and Method for Treating a Tachyarrhythmia” toKeimel, for example, both incorporated herein by reference in theirentireties, for selectively delivering single phase, simultaneousbiphasic and sequential biphasic cardioversion-defibrillationstimulation therapy. In an exemplary implementation, IMD 14 may deliverstimulation therapy employing housing electrode 36 coupled to theterminal HV-A and at least one electrode such as electrode 32 coupled tothe node HV-B output (at terminals 36 a and 32 a, respectively) of theoutput circuit 56. In alternative embodiments, the IMD 14 may employadditional electrodes such as electrodes 30, 34 coupled to nodes such asS1, S2 (at terminals 30 a and 34 a, respectively) for sensing orstimulation therapy.

The cardioversion-defibrillation stimulation therapy energy andcapacitor charge voltages can be intermediate to those supplied by ICDshaving at least one cardioversion-defibrillation electrode in contactwith the heart and most AEDs having cardioversion-defibrillationelectrodes in contact with the skin. The typical maximum voltagenecessary for ICD 14 using most biphasic waveforms is approximately 750Volts with an associated maximum energy of approximately 40 Joules. Thetypical maximum voltage necessary for AEDs is approximately 2000-5000Volts with an associated maximum energy of approximately 200-360 Joulesdepending upon the waveform used. The SubQ ICD 14 of the presentdisclosure uses maximum voltages in the range of about 700 to about 3150Volts and is associated with energies of about 25 Joules to about 210Joules. The total high voltage capacitance could range from about 50 toabout 300 microfarads.

Such cardioversion-defibrillation stimulation therapies are onlydelivered when a malignant tachyarrhythmia, e.g., ventricularfibrillation is detected through processing of the far field cardiac ECGemploying one of the available detection algorithms known in the ICD 14art.

In FIG. 4, pacer timing/sense amplifier circuit 52 processes the farfield ECG SENSE signal that is developed across a particular ECG sensevector defined by a selected pair of the electrodes 36, 32, andoptionally, electrodes 30, 34 if present as noted above. The selectionof the sensing electrode pair is made through a control circuit 54 in amanner to provide the most reliable sensing of the EGM signal ofinterest, which would be the R wave for patients who are believed to beat risk of ventricular fibrillation leading to sudden death. The farfield ECG signals are passed through the control circuit 54 to the inputof a sense amplifier in the pacer timing/sense amplifier circuit 52.

Control circuit 54 may comprise one or more microprocessors,Application-Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Field-Programmable Gate Arrays (FPGAs), discreteelectronic components, state machines, sensors, and/or other circuitry.Control circuit 54 may operate under the control of programmedinstructions such as software and/or firmware instructions stored withina storage device (70). The storage device may include volatile,non-volatile, magnetic, optical, and/or electrical media for storingdigital data and programmed instructions, including Random Access Memory(RAM), Read-Only Memory (ROM), Non-Volatile RAM (NVRAM), ElectricallyErasable Programmable ROM (EEPROM), flash memory, removable storagedevices, and the like. These one or more storage devices 70 may storeprograms executed by control circuit 54.

Storage devices 70 may likewise store data, which may include, but isnot limited to, programmed parameters, patient information, data sensedfrom the patient, and status information indicating the status of theICD 14. For instance, the data may include statistical information andother characteristic data about the battery (or individual cell) that isused to predict charge remaining within the power source 46 of ICD 14 aswill be discussed in more detail below. The data may further contain ERIand/or EOL indicators to indicate when replacement operations will beneeded. This information may be provided to a clinician or patient viathe external device 4.

Detection of a malignant tachyarrhythmia is determined via the controlcircuit 54 as a function of one or more sensed signals (e.g., R-wavesignals and/or P-wave signals) that are output from the pacertiming/sense amplifier circuit 52 to the control circuit 54. An exampledetection algorithm is described in U.S. Pat. No. 7,103,404, titled“Detection of Tachyarrhythmia Termination”, issued to Stadler, which isincorporated herein by reference in its entirety. Certain steps in theperformance of the detection algorithm criteria are cooperativelyperformed in a microcomputer 50, including stored detection criteriathat may be programmed into via a telemetry interface (not shown)conventional in the art.

The microcomputer 50 is generally representative of a processor andassociated memory in storage device 70. The memory may reside internallywithin the microcomputer 50, or separately in storage device 53. Thememory, for example, may include computer readable instructions that,when executed by processor, cause the operational circuitry and or anyother component of the medical device to perform various functionsattributed to them. For example, the memory may include any volatile,non-volatile, magnetic, optical, or electrical media, such as a randomaccess memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM),electrically-erasable programmable ROM (EEPROM), flash memory, or anyother digital media. Such memory will typically be non-transitory. Theprocessor, may include any one or more of a microprocessor, a digitalsignal processor (DSP), a controller, an application specific integratedcircuit (ASIC), a field programmable gate array (FPGA), or equivalentdiscrete or integrated logic circuitry. In one or more exemplaryembodiments, the processor may include multiple components, such as anycombination of one or more microprocessors, one or more controllers, oneor more DSPs, one or more ASICs, or one or more FPGAs, as well as otherdiscrete or integrated logic circuitry. The functions attributed to themicrocomputer 50 may be embodied as software, firmware, hardware, or anycombination thereof.

Data and commands are exchanged between microcomputer 50 and controlcircuit 54, pacer timing/amplifier circuit 52, and output circuit 56 viaa bi-directional data/control bus 61. The pacer timing/amplifier circuit52 and the control circuit 54 are clocked at a slow clock rate. Themicrocomputer 50 is normally asleep, but is awakened and operated by afast clock by interrupts developed by sensed cardiac events or onreceipt of a downlink telemetry programming instruction or upon deliveryof cardiac pacing pulses to perform any necessary mathematicalcalculations, to perform tachycardia and fibrillation detectionprocedures, and to update the time intervals monitored and controlled bythe timers in pace/sense circuitry 52.

The detection algorithms are highly sensitive and specific for thepresence or absence of life threatening ventricular arrhythmias, e.g.,ventricular tachycardia (V-TACH) and ventricular fibrillation (V-FIB).As discussed above, the detection algorithms contemplated in accordancewith this disclosure may utilize sensed cardiac signals to detect thearrhythmias. In addition, detection algorithms for atrial fibrillationmay also be included.

Although the ICD 14 of the present disclosure may rarely be used for anactual sudden death event, the simplicity of design and implementationallows it to be employed in large populations of patients at modest riskwith modest cost by medical personnel other than electrophysiologists.Consequently, the ICD 14 of the present disclosure includes theautomatic detection and therapy of the most malignant rhythm disorders.

When a malignant tachycardia is detected, high voltage capacitors (notshown) within the output circuit are charged to a pre-programmed voltagelevel by a charging circuit 58. It is generally considered inefficientto maintain a constant charge at all times on the high voltagecapacitors. Instead, charging is initiated when control circuit 54issues a high voltage charge command delivered to charging circuit 58and charging is controlled by means of bi-directional signal line(s)from the HV output circuit 56. Without intending to be limiting, thehigh voltage output capacitors may comprise film, aluminum electrolyticor wet tantalum construction. Some examples of the high voltage outputcapacitors are described in commonly assigned U.S. Pat. No. 8,086,312,titled “Capacitors for Medical Devices”, issued to Nielsen, which isincorporated herein by reference in its entirety.

The high voltage output capacitors may be charged to very high voltages,e.g., 700-3150V, to be discharged through the body and heart between theselected electrode pairs among first, second, and, optionally, thirdand/or fourth subcutaneous cardioversion-defibrillation electrodes 36,32, 30, 32. The details of an exemplary charging circuit 58 and outputcircuit 56 will be discussed below. The high voltage capacitors arecharged by charging circuit 58 and a high frequency, high-voltagetransformer. The state of capacitor charge is monitored by circuitrywithin the output circuit 56 that provides a feedback signal indicativeof the voltage to the control circuit 54. Control circuit 54 terminatesthe high voltage charge command when the received signal matches theprogrammed capacitor output voltage, i.e., thecardioversion-defibrillation peak shock voltage.

Control circuit 54 then develops a control signal that is applied to theoutput circuit 56 for triggering the delivery of cardioverting ordefibrillating shocks. In this way, control circuitry 54 serves tocontrol operation of the high voltage output stage 56, which delivershigh energy cardioversion-defibrillation stimulation therapies between aselected pair or pairs of the first, second, and, optionally, the thirdand/or fourth cardioversion-defibrillation electrodes 36, 32, coupled tothe HV-A, HV-B and optionally to other electrodes such as electrodes 34,30 coupled to the S1, S2 terminals as shown in FIG. 4.

Thus, ICD 14 monitors the patient's cardiac status and initiates thedelivery of a cardioversion-defibrillation stimulation therapy through aselected pair or pairs of the first, second, third and/or fourthelectrodes 36, 32, 34, and 30 in response to detection of atachyarrhythmia requiring cardioversion-defibrillation.

Typically, the charging cycle of the capacitors has a short duration,e.g., it can take anywhere from two seconds to about thirty seconds, andoccurs very infrequently. The ICD 14 can be programmed to attempt todeliver cardioversion shocks to the heart in the manners described abovein timed synchrony with a detected R-wave or can be programmed orfabricated to deliver defibrillation shocks to the heart in the mannersdescribed above without attempting to synchronize the delivery to adetected R-wave. Episode data related to the detection of thetachyarrhythmia and delivery of the cardioversion-defibrillationstimulation therapy can be stored in RAM for uplink telemetrytransmission to an external programmer as is well known in the art tofacilitate in diagnosis of the patient's cardiac state.

Housing 14 may include a telemetry circuit (not shown in FIG. 4), sothat it is capable of being programmed by means of external device 4(FIG. 1) via a 2-way telemetry link. Uplink telemetry allows devicestatus and diagnostic/event data to be sent to external programmer forreview by the patient's physician. Downlink telemetry allows theexternal programmer via physician control to allow the programming ofdevice function and the optimization of the detection and therapy for aspecific patient. Programmers and telemetry systems suitable for use inthe practice of the present disclosure have been well known for manyyears. Known programmers typically communicate with an implanted devicevia a bi-directional telemetry link such as Bluetooth®, radio-frequency,near field, or low frequency telemetry link, so that the programmer cantransmit control commands and operational parameter values to bereceived by the implanted device, and so that the implanted device cancommunicate diagnostic and operational data to the programmer.

Those skilled in the art will appreciate that the various components ofthe low power circuit 60 i.e., pacer/sense circuit 52, control circuit54, and microcomputer 50 are illustrated as separate components for easeof discussion. In alternative implementations, the functions attributedto these components 50, 52 and 54 may suitably be performed by a solecomponent.

As mentioned above, the control circuit 54 and output circuit 56performs several functions. One of those is to monitor the state ofcapacitor charge of the high voltage output capacitors. Another functionis to allow the controlled transfer of energy from the high voltageoutput capacitors to the patient.

FIG. 5 illustrates an exemplary schematic showing a portion of theoperational circuitry 48 of FIG. 4, in accordance with an embodiment ofthe disclosure, in greater detail. The output circuit 56 allows thecontrolled transfer of energy from the energy storage capacitors to thepatient 12.

The output circuit 56 includes four legs 80, 82, 84, and 86 that areinterconnected. The interconnection of the four legs with legs 80 and 82being configured in a parallel orientation alongside legs 84 and 86 anda bridge being provided to intersect each of the pair of parallelconnected legs. As is shown in FIG. 5, the interconnected legs arearrayed to define a configuration includes a high side and a low sidethat may resemble a “H”. In other words, the four interconnected legsare arrayed having legs 80 and 84 defining the high side and legs 82 and86 defining the low side.

The intersecting bridge includes HV-A and HV-B terminals that couple theoutput circuit 56 to the cardioversion electrodes 36 and 32. Aspreviously described, patient 12 is connectable (e.g., usingleads/electrodes 36, 32 and any other suitable connections) betweenterminal HV-A located between the switch 80 and switch 82 and terminalHV-B located between switch 84 and switch 86.

Legs 80 and 84 are coupled to a positive terminal of the energy storagecapacitors. An optional discharge switch 88, such as an insulated gatebipolar transistor (IGBT), may be used in the coupling from the legs 80and 84 to the positive terminal of the energy storage capacitors.Discharge switch 88 may be controlled by control circuit 54 (FIG. 4)that is included within the low power circuit 60 to close and remain inthe conducting state during discharge of the capacitors. Leg 82 and 86are coupled to a negative terminal of the energy storage capacitors. Theselection of one or more of the switches 80, 82, 84, 86 under control ofcontrol circuit 54 may be used to provide one or more functions. Forexample, selection of certain switches in one or more configurations maybe used to provide one or more types of stimulation pulses, or may beused to provide active or passive recharge, etc.

For example, in accordance with an embodiment, the ICD 14 provides abiphasic defibrillation pulse to the patient in the following manner.With reference to FIG. 5, once the energy storage capacitors are chargedto a selected energy level, the switches 80, 86, and 88 are closed so asto provide a path from the capacitors to electrode 36, 32 for theapplication of a first phase of a defibrillation pulse to the patient12. The stored energy travels from the positive terminal of thecapacitors, through switch 88 through switch 80, across the patient 12,back through switch 86 to the negative terminal of the capacitors. Thefirst phase of the biphasic pulse therefore applies a positive pulsefrom the electrode 36 to the electrode 32.

After the end of the first phase of the biphasic defibrillation pulse,the switches 88, 84 and 82 are switched on to start the second phase ofthe biphasic pulse. Switches 84 and 82 provide a path to apply anegative defibrillation pulse to the patient 12. With reference to FIG.5, the energy travels from the positive terminal of the capacitors,through switch 88 to switch 84, across the electrodes 32, 36 coupled tothe patient 12, and out through switch 82 to the negative terminal ofthe capacitors. The polarity of the second phase of the defibrillationpulse is therefore opposite in polarity to the first phase of the pulse.

FIGS. 6 and 7 are schematic diagrams illustrating a portion of theoperational circuit 48 of IMD 14. For ease of discussion, the elementsthat are common to both FIGS. 6 and 7 are numbered with identicalreference designators.

As previously mentioned, the operational circuit 48 includes at leastone power source 46. The power source 46 may comprise a battery havingat least two cells 102 a, 102 b (collectively “102”). In exemplaryembodiments, the power source 46 may be programmable or static and maybe a switched or linear regulated source, etc. In an embodiment, thecells 102 supply power to the operational circuit 48 as well as thecharge for stimulation therapy energy. The cells 102 may be formed frommaterials such as LiCFx, LiMnO2, LiI2, LiSVO or LiMnO2, among others, asis known in the art.

Each of the cells 102 is coupled to a transformer 64 that is includedwithin the output circuit 56 (shown in dashed lines in FIGS. 6 and 7).In an embodiment, the transformer 64 may be configured as a dual primarytransformer having a first primary winding 106 a and a second primarywinding 106 b. In the embodiment, the cell 102 a is coupled to the firstprimary winding 106 a and the cell 102 b is coupled to the secondprimary winding 106 b.

A first switching element 108 a is coupled between the first primarywinding 106 a of the transformer and the cell 102 a. A second switchingelement 108 b is coupled between the second primary winding 106 b of thetransformer 64 and the cell 102 b. Although not shown in FIGS. 6 and 7,each of the switches 108 a, 108 b is coupled to a control circuit, suchas control circuit 54 (FIG. 4), which issues control signals (CS1, CS2)to selectively actuate each of the switches 108 a, 108 b. The controlsignals may be issued to selectively actuate the switching elements 108a, 108 b separately, simultaneously or in any other desired manner.

The cells 102 may be formed such that each cell includes a cathode(positive) terminal and an anode (negative) terminal. As is illustratedin the depicted embodiment, the cathode terminals of cells 102 a, 102 bare coupled to the primary winding 106 a and the primary winding 106 b,respectively. The switches 108 a, 108 b may be coupled to the commonnode as shown in FIG. 6. As such, a first circuit path is definedbetween the first cell 102 a and first primary winding 106 a and asecond circuit path is defined between the second cell 102 b and theprimary winding 106 b.

In FIG. 6, the anode terminals of cells 102 a, 102 b are both connectedto a common node, such as the circuit ground node. In the embodiment ofFIG. 7, each of the anode terminals of the cells 102 a, 102 b is coupledto separate ground nodes such as GND1 and GND2. In FIG. 7, the switches108 a, 108 b are coupled to the separate ground node GND1 and GND2,respectively.

In one embodiment, the switches 108 are simultaneously actuated to aconducting state to enable current to flow from both cells 102 to thetransformer 64. The actuation of the first switch 108 a into a closedposition triggers charge transfer from the first cell 102 a to the firstprimary winding 106 a and actuation of the second switch 108 b into aclosed position triggers charge transfer from the second cell 102 b tothe second primary winding 106 b. In other words, the closing of switch108 a creates a current path for flow of current from the first cell 102a to the transformer 106 a while the closing of switch 108 b creates acurrent path for flow of current from the second cell 102 b to thetransformer 106 b.

An isolation circuit is coupled to the first cell 102 a and to thesecond cell 102 b. As will be described in greater detail with referenceto FIGS. 6 and 7, the isolation circuit is configured to maintain acurrent isolation between the first cell 102 a and the second cell 102b. The current isolation functionality provided by the isolation circuitmay occur throughout the operating life of the cells 102.

In the embodiment of FIG. 6, an isolation circuit 110 a may comprise aresistor that is selected having a value that enables current to flowfrom the first cell 102 a to the low power circuitry 60 during low powercurrent operations, but does not allow high power current to flowbetween the cells. Without intending to be limiting, low power currentoperations may include operations associated with the analog and digitalportions of the operational circuitry 48 while the high power currentoperations may include generation of electrical stimulation therapy thatis delivered to the patient 12 based on a treatment regimen, as is knownin the art. As such, the resistor value will be large enough to preventflow of high power current between cell 102 a and cell 102 b during highpower operations such as high clock speed operations like telemetry, butyet low enough to allow flow of low power current between the cell 102 aand cell 102 b during low power operations such as low clock speedoperations like data storage operations. For example, the resistor mayhave a value in the range of 10 Ohms to 10,000 Ohms. In another example,the resistor may have a value in the range of 500 Ohms to 1,500 Ohms. Inthe embodiment of FIG. 6, the isolation circuit 110 a is coupled alongthe current pathway from the cathode of cell 102 a to the low powercircuit 60 and transformer 106 b.

Turning to the embodiment of FIG. 7, an isolation circuit 110 b maycomprise a resistor that is selected having a value that enables currentto flow from the first cell 102 a to the low power circuitry 60 duringlow power current operations, but does not allow high power current(e.g., current delivered during high power operations) to flow betweenthe cells. For example, the resistor may have a value in the range of500 Ohms to 1500 Ohms. In the embodiment of FIG. 7, the isolationcircuit 110 b is coupled along the current pathway from the anode ofcell 102 a to the low power circuit 60 and the switch 108 a. Thus, theground node for the low power circuit 60 is coupled to either GND1 orGND2.

With reference again to FIGS. 6 and 7, each of the isolation circuits110 a, 110 b (collectively “isolation circuit 110”) depicted thereinprovides current isolation between the first cell 102 a and second cell102 if the current level exceeds a predetermined threshold, whileallowing both cells to contribute to operations of the low powercircuitry 60 that are powered by power supply having a current levelthat is at or below the predetermined threshold. In the event of afailure of one of the cells, for example, the impact of the failure ismitigated by preventing the other cell from discharging into the failedcell. As shown in FIGS. 6 and 7, the cells 102 are arranged in aparallel configuration and the isolation circuit 110 is coupled to oneterminal of each of the cells 102 a, 102 b.

The low power circuit 60 receives a first level of power from the firstand second cells 102, while a high power circuit receives a second levelof power (through first and second primary windings 106 of transformer64) from the first and second cells 102. The first level of powersupports low power current operations and is less that the second levelof power. The second level of power supports high power currentoperations. In other words, the current delivered by the first andsecond cells 102 to the low power circuit 60 is less than the currentdelivered to the high power circuit.

As such, the isolation circuit 110 is configured to maintain high powercurrent isolation between the first cell 102 a and to the second cell102 b while allowing each cell 102 a and cell 102 b to contribute tohigh power current operation through the first primary winding 106 a andthe second primary winding 106 b, respectively, and allow both cells toconcurrently deliver power to the low power circuitry 60 of theoperational circuitry 48 such as 50, 52, and 54 for low power currentoperations. In other words, the isolation circuit 110 provides currentisolation between the first cell 102 a and second cell 102 b if thecurrent level exceeds a predetermined threshold, while allowing bothcells to contribute to operations of the low power circuitry 60 that arepowered by power supply 46 having a current level that is at or belowthe predetermined threshold. As such, in the event of a failure of oneof the cells, for example, the impact of the failure is mitigated bypreventing the other cell from discharging into the failed cell. Thehigh power current operations include the delivery of energy to thetransformer 64 to, for example, provide defibrillation therapy. The lowpower current operations include supply of power to low power circuitry60. For simplicity of description, the interconnections between thecells 102 and all the components of the operational circuit 48 is notshown. In the event of a failure of one of the cells 102, the isolationcircuit 110 isolates the failed cell from the other cell.

The cell 102 b is directly coupled to the low power circuitry 60 whilethe second cell 102 a is coupled to the low power circuitry 60 throughthe isolation circuit 110. As previously discussed, the low powercircuitry 60 may also include power source monitoring circuitry 62coupled to the cells 102 to monitor the energy level of each cell. Anyknown battery monitoring techniques, such as coulomb counting or adirect voltage measurement may be utilized to monitor the state ofcharge of the individual cells. In this embodiment, the monitoring ofthe cells 102 may be performed through monitoring lines MS1, MS2.

Providing software, firmware and hardware to accomplish the presentinvention, given the disclosure herein, is within the abilities of oneof skill in the art. For the sake of brevity, conventional techniquesrelated to ventricular/atrial pressure sensing, IMD signal processing,telemetry, and other functional aspects of the systems (and theindividual operating components of the systems) may not be described indetail herein. The connecting lines shown in the various figurescontained herein are intended to represent example functionalrelationships and/or physical couplings between the various elements. Itshould be noted that many alternative or additional functionalrelationships or physical connections may be present in an embodiment ofthe subject matter.

The description refers to elements or nodes or features being“connected” or “coupled” together. As used herein, unless expresslystated otherwise, “connected” means that one element/node/feature isdirectly joined to (or directly communicates with) anotherelement/node/feature, and not necessarily mechanically. Likewise, unlessexpressly stated otherwise, “coupled” means that oneelement/node/feature is directly or indirectly joined to (or directly orindirectly communicates with) another element/node/feature, and notnecessarily mechanically. Thus, although the schematics shown in thefigures depict exemplary arrangements of elements, additionalintervening elements, devices, features, or components may be present inan embodiment of the depicted subject matter.

While the disclosure is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the disclosure to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the disclosure as defined by the appended claims.

What is claimed is:
 1. An implantable medical device, comprising: a highpower output circuit for delivery of an electrical stimulation therapy;a first cell directly coupled to the high power output circuit; a secondcell directly coupled to the high power output circuit, the first andsecond cells being arranged in a parallel configuration; a low powercontrol circuit directly coupled to the second cell, wherein the lowpower circuit receives a first level of current from the first andsecond cells; and an isolation circuit coupled to the first cell, thesecond cell, and the low power control circuit, wherein the isolationcircuit is configured to maintain a current isolation between the firstcell and the second cell such that a second level of current isdelivered from both the first and second cells to the high power outputcircuit, wherein the second level of current is greater than the firstlevel of current.
 2. The implantable medical device of claim 1, whereinthe first cell comprises a first cathode and the second cell comprises asecond cathode and the isolation circuit is coupled between the firstcathode and the second cathode, and wherein the low power circuit iscoupled to the first cathode through the isolation circuit, and the lowpower circuit is directly coupled to the second cathode.
 3. Theimplantable medical device of claim 1, wherein the isolation circuit isconfigured to maintain a current isolation between the first cell andthe second cell during the delivery of charge from both the first andsecond cells to the high power output circuit.
 4. The implantablemedical device of claim 1, wherein the isolation circuit is configuredto isolate the first cell from the second cell in the event of a failureof one of the first or second cells.
 5. The implantable medical deviceof claim 1, further comprising a first switching element coupled along acurrent path defined between the first cell, the high power outputcircuit and a common node shared by the first cell and the second cell,wherein the first switching element selectively couples the first cellto the high power output circuit for delivery of charge.
 6. Theimplantable medical device of claim 5, further comprising a secondswitching element coupled along a current path defined between thesecond cell, the high power output circuit and the common node shared bythe first cell and the second cell, wherein the second switching elementselectively couples the second cell to the high power output circuit fordelivery of charge.
 7. The implantable medical device of claim 6,wherein the low power control circuit controls the actuation of each ofthe first and second switching elements to simultaneously open or closethe first and second switching elements for selective coupling of thefirst and second cells to the high power output circuit for delivery ofcharge.
 8. The implantable medical device of claim 5, wherein the commonnode comprises a circuit ground.
 9. The implantable medical device ofclaim 1, wherein the high power output circuit comprises a transformerhaving a first primary winding and a second primary winding, the firstcell being directly coupled to the first primary winding and the secondcell being directly coupled to the second primary winding.
 10. Theimplantable medical device of claim 1, wherein the isolation circuitcomprises a resistive load.
 11. The implantable medical device of claim10, wherein the resistive load comprises an impedance value between 500ohms and 1500 ohms.
 12. The implantable medical device of claim 1,wherein the low power control circuit comprises a monitoring circuit formonitoring a state of charge of each of the first and second cells. 13.The implantable medical device of claim 12, wherein the monitoringcircuit monitors the state of charge through coulomb counting.
 14. Theimplantable medical device of claim 12, wherein the monitoring circuitmonitors the state of charge through voltage measurement.
 15. Theimplantable medical device of claim 1, wherein the high power outputcircuit comprises a first transformer and a second transformer, thefirst cell being directly coupled to the first transformer and thesecond cell being directly coupled to the second transformer.
 16. Theimplantable medical device of claim 1, wherein the isolation circuitcomprises a resistive load having a value in the range of 10 to 10000ohms.
 17. The implantable medical device of claim 1, wherein the firstcell comprises a first anode and the second cell comprises a secondanode, wherein the isolation circuit is coupled between the first anodeand the second anode, and the low power circuit is coupled to the firstanode through the isolation circuit, and wherein the second anode isdirectly coupled to the low power circuit.
 18. The implantable medicaldevice of claim 17, wherein the first cell comprises a first cathode andthe second cell comprises a second cathode and the high power outputcircuit is directly coupled to the first and second cathodes.
 19. Theimplantable medical device of claim 17, wherein the first cell iscoupled to a first ground and the second cell is coupled to a secondground that is different from the first ground.